1. Field of the Invention
The present invention relates to a configuration control method for making use of the redundant loop transmission line of a double loop communications system which is able to handle simultaneously two types of traffic mode, namely, packet-switched data and circuit-switched data.
2. Description of the Prior Art
A double loop communications system using a conventional communications system based on the fiber distributed data interface type II (FDDI-II), which is being standardized by the American National Standards Institute (ANSI), is shown in FIG. 3. The communication system includes the first loop transmission lines 1a, 1b, . . ., 1n; the second loop transmission lines 2a, 2b, . . ., 2n; and stations 3a, 3b, . . ., 3n. Each station 3 includes a first transmission line interface circuit 4 for performing serial/parallel conversion and encoding/decoding of data; and a second transmission line interface circuit 5 having the same function as the interface circuit 4; a hybrid multiplexer 6 for controlling a periodic transmission frame which is transmitted through the loop transmission line at a certain period and separating the packet data and circuit switched data from the periodic transmission frame or multiplexing them; a token passing control circuit 7 for controlling, by token passing access control, transmission or transfer of the packet data in the packet bandwidth selected by the hybrid multiplexer 6; a circuit-switched channel control circuit 8 for controlling circuit switched channel (or transfer of the circuit switched data) in the circuit-switched bandwidth selected by the hybrid multiplexer 6; selectors 9-11 for selecting respective inputs; and signal lines 12-21.
In operation, in the normal operation wherein all the loop transmission lines are available, the serial data inputted via the first loop transmission line 1 is converted into parallel data and decoded by the first transmission line interface circuit 4 and transmitted to the selector 11 via the signal line 12. The signal line 12 is selected in the selector 1 and connected to the signal line 16. The data from the signal line 16 is identified with the control, packet, and circuit-switched bandwidth by the hybrid multiplexer 6. The data stream in the packet bandwidth is transferred to the token passing control circuit 7 via the signal line 18 while the data stream in the circuit-switched bandwidth is transferred to the token passing control circuit 7 via the signal line 18 while the data stream in the circuit-switched bandwidth is transferred to the circuit-switched channel control circuit 8 via the signal line 20.
The data streams sent by the token passing control circuit 7 and the circuit-switched channel control circuit 8, on the other hand, are sent to the hybrid multiplexer circuit 6 via the signal lines 19 and 21, respectively. These two data streams are allocated to predetermined bandwidth in the hybrid multiplexer 6 to form a transmission frame, which is sent onto the signal line 17. The selector 9 connects the signal line 17 to the signal line 13. The first transmission line interface circuit 4 encodes the data stream carried by the signal line 13 and converts it into serial data and sends it to the downstream station via the first loop transmission line 1. The data carried by the second loop transmission line 2a is sent to the second loop transmission line 2n via the transmission line interface circuit 5, the signal line 14, the selector 10, the signal line 15, and the transmission line interface circuit 5.
The operation in the event of fault occurrence will be described. For example, when a fault occurs on a loop transmission line either 1b or 2b, thus disabling data transfer, the stations 3b and 3c perform loopback control, thereby enabling communications again. A case wherein the loop transmission line 1b is broken will be discussed below. First of all, the transmission line interface circuit 4 of the station 3c detects an input signal interruption. In response to instructions from the control unit (not shown), the selector 11 of the station 3c selects the signal line 14, while the transmission line interface circuit 5 stops sending a signal to the loop transmission line 2b. The transmission line interface circuit 5 of the station 3b then detects an input signal interruption on the loop transmission line 2b. In response to instructions from the control unit, the selector 10 switches from the signal line 14 to select the signal line 17. As a result, the data carried by the loop transmission line lb is now carried by the loop transmission line 2a.
As has been described above, in normal operation, only one of the double loop transmission lines is used, making it difficult to increase the system throughput. Even if both the loop transmission lines are used for the packet data and the circuit-switched data, respectively, only the packet data is valid in the event of fault occurrence.